“万一”用英语怎么说

如果搜词典，“万一”通常给的翻译是 in case。但是 in case 的意思是“要是这种情况发生了”的“要是”，它并不能体现这件事不太可能发生。

另一个比较贴近的表达是 If + were to

In formal situations, we can use if + were to when we talk about things that might happen but which we think are unlikely.

这就有了“不太可能”这个意思在里面。例句：

If the Prime Minister were to resign, there would have to be a general election within 30 days.